I was getting this issue as well after following the same tutorial. I found I had an older version of redis previously installed that had created a redis-server.service file which specified an alias for itself of redis.service.

I ran sudo apt-get purge –auto-remove redis-server and then ran back through the tutorial steps. It’s possible I could have gotten away with just re-inserting my redis.conf file, since it was gone after the purge. But given that I redid everything, I couldn’t say for certain.